In various applications of inductive coupling communication, RFID systems are used which, for example, are operated at 13.56 MHz. Communication in these systems is done by magnetic coupling between an RFID reader coil and an RFID tag coil. RFID tags are passive devices, which for example are composed of an integrated circuit and some kind of antenna that is attached to terminals of the integrated circuit. For example, data are transmitted from the tag to the reader using a load modulation. In other implementations, active transmission is used for transmitting data. Examples for systems with active transmission can be found in WO 2013/002736 A1.
U.S. Patent Appl. Pub. No. 2010/0245039 A1 shows a low frequency wake-up device with three receiver antennas, whose Q-factor can be adjusted, and with a separate transmitter antenna. In contrast to RFID systems, different antennas and frequencies are used for transmitting and receiving signals.
Active transmit systems need to synchronize their internal frequency source to the frequency and phase of a carrier signal emitted by the reader. This is usually done using a phase-locked loop, PLL, system which locks to the receive signal of the tag, in particular the induced signal resulting from the magnetic field of the reader. Accordingly, in conventional applications such a PLL is in a locked mode during a receive phase when the tag is not transmitting. When an active transmission of the tag starts, the amplitude of the transmit signal, in particular at the antenna of the tag, may be two orders of magnitude higher than the amplitude of the receive signal. This can effect that the receive signal may be completely obscured by the transmission signal.
Accordingly, the PLL preferably is operated in an unlocked state or free run state during this time. As it cannot be easily achieved that a free running PLL keeps its phase relation to a reader's carrier signal for a longer period of time, usually a resynchronization is performed in appropriate intervals. For example, the resynchronization is preferably performed in short time slots during a transmission of a data frame, when there is a pause in a transmit burst. However, due to the different oscillation amplitudes between transmission and reception of signals, and in particular the need for lower oscillation amplitudes, it is desirable to change the oscillation amplitude for resynchronization.